1. Field of the Invention
The present invention relates to UV curable inkjet compositions, and more specifically to UV curable inkjet inks, which are suitable for jetting by high-density print heads having small outer nozzle diameters.
2. Description of the Related Art
In inkjet printing, tiny drops of fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the ink-receiver or vice versa or both.
When jetting inkjet ink onto an ink-receiver, the ink typically includes a liquid vehicle and one or more solids, such as dyes or pigments and polymers. Ink compositions can be roughly divided in:
water-based, the drying mechanism involving absorption, penetration and evaporation;
solvent-based, the drying primarily involving evaporation;
oil-based, the drying involving absorption and penetration;
hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification; and
UV-curable, in which drying is replaced by polymerization.
It should be clear that the first three types of ink compositions are more suitable for an absorbing ink-receiver, whereas hot melt inks and UV-curable inks can also be printed on non-absorbing ink-receivers. Due to thermal requirements posed by hot melt inks on the substrates, especially UV curable inks have gained the interest of the industry in inkjet printing applications.
Industrial inkjet continues to require higher printing speeds and thinner image layers for UV curable inks. Thinner image layers lead to an improvement of flexibility and a lower production cost which can be obtained by allowing an ink droplet to spread on an ink receiver during a longer time period. However, this is disadvantageous for image quality. In order to achieve higher printing speeds and to maintain image quality, the fire frequency and/or nozzle density have to be increased.
Increasing the nozzle density leads to smaller nozzle diameters and thus smaller ink droplet volumes. For example, at 360 dpi the droplet volume is about 87 pL and leads to an ink layer thickness of 17.5 ml/m2. At 900 dpi, the droplet volume becomes 7.7 pL and the resulting ink layer thickness is only 9.7 mL/m2.
A problem is that smaller ink droplets exhibit a smaller droplet velocity because of relatively higher friction losses in a nozzle. The skilled person knows that droplet velocity can be increased by addition of organic solvent or the use of monofunctional monomers.
However, organic solvents tend to evaporate at the nozzles of an inkjet print head during a prolonged non-printing time. When restarting the printer, some nozzles appear to be clogged (=failing nozzles). This phenomenon is called latency. When high levels of organic solvents are employed, evaporation of such solvents in the drying process potentially presents both environmental and health and safety hazards. Using large amounts of monofunctional monomers in an ink generally exhibit a lower curing speed of the ink.
US 2009099277 (HEXION) discloses a radiation curable and jettable ink composition comprising an ethylenically unsaturated polyfunctional component and an ethylenically unsaturated monofunctional monomer, wherein the composition is substantially free of solvent.
U.S. Pat. No. 6,310,115 (AGFA) discloses radiation curable inkjet compositions for containing radiation curable polyfunctional monomers containing vinyl ether and acrylate functions.
Therefore, there exists a need in industrial inkjet printing for higher printing speeds and thinner image layers with UV curable inks, while maintaining good curing speed, image quality and latency.